The invention relates to a transport system for flat shipments, having a feeding part and a discharge part, in which the separated shipments are transported in clamped fashion, and a transport device of controllable speed, disposed between the two parts.
Particularly in distribution systems, the shipments are transported at high speeds and with minimal spacings therebetween as individual shipments are clamped between endless guide belts. Since the gaps between the shipments can differ because of shifting during transportation or because of doffing errors on being separated, correction of the gaps is necessary if the high performance and operation with little malfunction is to be maintained. If the shipments are to be transported at different speeds in the distributor machine, then when the speeds are changed, the gaps must absolutely be corrected if constant spacings between shipments are to be assured. That is, if the speed is increased or reduced, then correspondingly the spacings between the shipments are increased or decreased.
Versions have become known in which to increase the transport speed, two guide belt systems are disposed one after another, of which the rear system in terms of the transport direction transports at higher speed. Because of the abrupt change in speed at the transition point, the shipments are subjected to tensile stress. This increases the gaps, as a function of the shipment length and the various friction conditions between the shipments and the different guide belts.
For gap correction, the shipments are accelerated and braked again by means of pairs of rollers driven in a controlled fashion, so that they are displaced forward. Once again, the result is tensile stress on the shipments.
A synchronizing device for high-speed flat products to be conveyed is known (German Published, Examined Patent Application DE-AS 1 288 017), in which the products to be transported are delivered by a conveyor belt system to an intermediate belt segment and are discharged with a further conveyor belt system. The intermediate belt segment has a variable-speed drive mechanism, with which shifting, among the conveyed products relative to one another, or in other words shifting of gaps, is corrected in such a way that the products being conveyed can be delivered with fixed spacing to a mail distributing machine. Since the pieces of mail, or letters, are always in the lead, a targeted relative shift to the rear is effected on the intermediate belt segment. For reasons dictated by the process, this intermediate belt segment is relatively long. Since only one shipment at a time can be located on this long segment, necessarily wide gaps occur between the letters. If large and small letters one after another with comparatively small gaps are needed, then this object can be attained with this system.
The object of the invention is therefore to create a transport system for changing the speed of shipments of various lengths that are transported individually and in succession with small gaps therebetween between them, and in a clamped fashion where arbitrary changes can be made without tensile or compressive stresses on the shipments.
By using a transport device which is upstream of the transport device that has the controllable drive mechanism and which does not hinder the accelerating transport of the shipments by the second transport device, no stresses are put on the shipments upon acceleration or braking, since in this period of time, the drive of the shipments is effected only by the transport device having the controllable drive mechanism.
With a view to a simple design, it is especially advantageous to embody the first transport device with a clamping point that can be engaged and disengaged.
It is also advantageous according to the invention to make the drive mechanism of the first transport device capable of being coupled and uncoupled.
In a further advantageous feature in accordance with the invention, the first transport device has a controllable drive, which, while the applicable shipment is clamped in the second transport device, is driven at the same speed as the second transparent device.
To reduce the switching operations of the drive of the first transport device, in an advantageous embodiment of the invention, this drive has a free-wheeling mechanism.
If the shipment in the second transport device is driven at a higher speed than in the first transport device, in which the shipment is still clamped, then the shipment can be transported with very little stress at the higher speed, since the free-wheeling mechanism makes the first transport device run at this speed as well.
For correcting gaps, the speed is advantageously changed in two acceleration phases in accordance with the invention. In the first phase, the applicable shipment is assigned a positive or negative acceleration, depending on the necessary displacement direction, and in the second phase, an adaptation to the speed of the downstream part of the transport system is made. The acceleration times and levels are defined in such a way that taking into account the available time and the length of the correction path, the necessary displacement is ended upon the transfer of the shipment to the downstream part of the transport system, and for the sake of handling the shipments gently, the accelerations are kept as slight as possible.
In an advantageous embodiment according to the invention, the correction path is made long enough that a maximum acceleration is not exceeded when the corrections are made.